This invention relates to a process for the preparation of isocyanates by reaction of corresponding primary amine(s) with phosgene in the gas phase. In this process, the reaction is terminated by guiding the reaction mixture out of the reaction chamber through a cooling stretch into which liquids are injected. Direct cooling takes place in the cooling stretch in one stage in two or more cooling zones connected in series.
Various processes for the preparation of isocyanates by reaction of amines with phosgene in the gas phase are known from the prior art. The advantages of these procedures are that intermediates that are difficult to phosgenate are avoided, high reaction yields can be achieved, the phosgene hold-up is reduced, and the amount of energy specifically required for the preparation of the isocyanates is relatively small.
EP-A-593 334 describes a process for the preparation of aromatic diisocyanates in the gas phase, characterised in that the reaction of the diamine with phosgene takes place in a tubular reactor without moving parts and with a narrowing of the walls along the longitudinal axis of the tubular reactor. However, the process is problematic because mixing of the starting material streams solely via a narrowing of the tube wall is poor as compared with the use of a proper mixing member. Poor mixing conventionally leads to undesirably high solids formation.
EP-A-699 657 describes a process for the preparation of aromatic diisocyanates in the gas phase, characterised in that the reaction of the associated diamine with the phosgene takes place in a two-zone reactor, wherein the first zone, which accounts for approximately from 20% to 80% of the total reactor volume, is ideally mixed and the second zone, which accounts for from 80% to 20% of the total reactor volume, can be characterised by a piston flow. However, because at least 20% of the reaction volume is ideally back-mixed, non-uniform dwell time distribution results, which can lead to undesirably increased solids formation.
EP-A-289 840 describes the preparation of diisocyanates by gas-phase phosgenation in a turbulent flow at temperatures of from 200° C. to 600° C. in a cylindrical chamber without moving parts. Because there are no moving parts, the risk of phosgene escaping is reduced. Due to the turbulent flow in the cylindrical chamber (tube), relatively good equipartition of flow in the tube is achieved, and accordingly a relatively narrow dwell time distribution is obtained. If fluid elements in the vicinity of the wall are disregarded, this can, as described in EP-A-570 799, lead to a reduction in solids formation.
EP-A-570 799 discloses a process for the preparation of aromatic diisocyanates in which the reaction of the associated diamine with the phosgene is carried out in a tubular reactor, above the boiling point of the diamine, within a mean contact time of from 0.5 to 5 seconds. As is described in the specification, both too long and too short a reaction time lead to undesirable solids formation. A process is therefore disclosed in which the mean deviation from the mean contact time is less than 6%. The contact time is observed by carrying out the reaction with a tubular flow that is characterised by either a Reynolds number above 4000 or a Bodenstein number above 100.
As already disclosed in EP-A-570 799, a common feature of all the processes known from the prior art for the preparation of isocyanates by reaction of amines with phosgene is that the isocyanates formed are not thermally stable at the reaction temperatures of from 300 to 600° C. that are conventionally used. It is therefore necessary to effectively terminate the reaction once an optimal reaction time has been reached in order to avoid the formation of undesirable secondary products by thermal decomposition of the isocyanate or by a further reaction.
To this end, in EP-A-0 289 840, the gaseous mixture that is continuously leaving the reaction chamber and that contains inter alia isocyanate that has formed, phosgene and hydrogen chloride is introduced into an inert solvent, for example dichlorobenzene. A disadvantage of this process is that the flow speed with which the gas mixture is passed through the solvent bath must be chosen to be relatively low because, at too high a speed, solvent and the compounds dissolved therein would be carried along. The liquid compounds would have to be separated from the gas in a subsequent step. A further disadvantage is that, owing to the low flow speeds and a poor heat transfer system, large solvent containers must be used to achieve cooling.
Also known are processes which use heat exchangers for cooling the reaction gases and/or expanding the gases in vacuo (DE 101 58 160 A1). The disadvantage of heat exchangers is that, because of the poor heat transfer, large exchange surfaces and accordingly large heat exchangers are required for effective cooling. In addition, solids are deposited on the comparatively cold surfaces of the heat exchangers as a result of secondary reactions of the gas mixture on those surfaces, such as, for example, decomposition or polymerization. The heat transfer is further impaired as a result, leading to a longer dwell time and accordingly a further increase in secondary product formation. Moreover, cleaning of the cooling stage results in undesirable stoppage times for the installation as a whole.
According to the teaching of EP-A-1 403 248, the problem of rapidly cooling the gaseous reaction mixture in the gas-phase phosgenation of amines with phosgene to a temperature at which the reaction product in question is thermally stable, while avoiding the mentioned disadvantages, and at the same time suppressing the formation of undesirable secondary products can be solved by cooling the reaction mixture leaving the reaction chamber in a single cooling zone by direct cooling while injecting a cooling liquid. EP-A-1 403 248 discloses a process for quenching a gaseous reaction mixture in the phosgenation of diamines in the gas phase to prepare diisocyanates in which the gaseous reaction mixture comprises at least one diisocyanate, phosgene and hydrogen chloride, by injecting a quench liquid into the gas mixture flowing continuously from a cylindrical reaction zone into the downstream cylindrical quench zone, the quench liquid being injected by means of at least two spray nozzles arranged at the entry to the quench zone at equal intervals along the periphery of the quench zone.
According to the teaching of EP-A-1 403 248, as well as containing phosgene, hydrogen chloride and the diisocyanate formed as the principal product, the gaseous reaction mixture can include other isocyanates, which are formed as secondary products, as well as nitrogen and/or organic solvents. As diisocyanates prepared by the gas-phase phosgenation of diamines, EP-A-1 403 248 discloses hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), naphthylene diisocyanate (NDI), toluene diisocyanate (TDI), diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate (HMDI). According to the teaching of EP-A-1 403 248, the advantage of the process is that, by the spraying of a suitable quench liquid, the desired rapid cooling of the gas mixture, which comprises a diisocyanate, hydrogen chloride and excess phosgene, as it leaves the reactor from 300 to 400° C. to a maximum of 150° C. is achieved. The contact time, in which cooling takes place, is from 0.2 to 3 seconds.
The direct cooling of the reaction mixture, disclosed in EP-A-1 403 248, during the preparation of isocyanates by the gas-phase phosgenation of corresponding amines with phosgene is likewise the subject-matter of WO 2005/123665. The cooling times of from 0.2 to 3 seconds disclosed in EP-A-1 403 248 lead, according to the teaching of WO 2005/123665, to a marked, avoidable loss of isocyanate. According to the teaching of WO 2005/123665, it is possible in the direct cooling of the reaction mixture of the preparation of isocyanates by the gas-phase phosgenation of corresponding amines with phosgene to achieve markedly shorter cooling times with a process in which the reaction of the amines with phosgene in the gas phase takes place in a reaction zone. In order to terminate the reaction, the reaction mixture is guided through a zone in which a liquid is injected. Between the reaction zone and the zone in which the liquid is injected, the reaction mixture is guided through a zone which has a reduced flow cross-section. According to the teaching of WO 2005/123665, the constriction of the flow cross-section is chosen so that the reaction mixture, on leaving the constriction, has been markedly cooled and possesses a high flow speed, which according to the teaching of WO 2005/123665 brings about effective “secondary atomisation” of the quench liquid. According to the teaching of WO 2005/123665, both requirements can be met if the Mach number of the flow in the constriction is from 0.1 to 1.0, preferably from 0.2 to 1.0, most preferably from 0.3 to 1.0. According to the teaching of WO 2005/123665, when the reaction gas stream emerging from the cross-sectional constriction at very high speed meets the quench liquid spray produced by means of single- or two-component atomizer nozzles having a Sauter diameter d23 of from 5 to 5000 μm, preferably from 5 to 500 μm, most preferably from 5 to 250 μm, it brings about “secondary atomisation” of the quench liquid, so that the spray has a particularly large specific surface area. According to the teaching of WO 2005/123665, the large specific surface area which can be achieved by the disclosed process, in conjunction with the high relative speed between the reaction gas and the quench liquid, causes an intensification of the exchange of material and heat between the reaction gas and the quench liquid, and the contact times required for cooling the reaction mixture are greatly reduced and the loss of valuable isocyanate product as a result of further reaction to secondary products is minimized. The necessary period of time between the entry of the reaction gas into the quench area and the time at which the reaction gas still differs by 10% from the adiabatic final temperature of the mixture of reaction gas and drops is disclosed in WO 2005/123665 as preferably from 10−4 to 10 seconds, more preferably from 5×10−4 to 1.0 seconds and most preferably from 0.001 to 0.2 seconds.
A disadvantage of the process disclosed in WO 2005/123665 is that the “secondary atomisation”, on which the process is based, of the quench liquid spray, which in the preferred range already consists of small droplets, involves the risk of mist formation with the consequence of a high outlay for separation of the isocyanate that has formed and the cooled reaction mixture. Another disadvantage is that, in the zone in which the cooling liquid is added, the high speed of emergence of the hot reaction mixture from the zone of reduced cross-section is to be taken into account by correspondingly increased apparatus dimensions.